1. Field of the Invention
The present invention relates to a method for removing phosphorus and nitrogen contained in sewage or wastewater using iron ore wastewater and, more particularly, to a method for removing phosphorus and nitrogen contained in sewage or wastewater by crystallizing the phosphorus and nitrogen in the form of struvite using iron ore wastewater containing a large amount of Mg2+ produced in a process of upgrading low-grade iron ore.
2. Discussion of Related Art
Recently, with the increase in the amount of nitrogen fertilizers, phosphorus fertilizers, etc. used due to soil degradation, a large amount of fertilizers are introduced into rivers, lakes, sea, etc., which promotes the growth of algae and causes eutrophication of rivers and sea. Moreover, a vicious circle in which the long-term accumulation of nitrogen fertilizers and phosphorus fertilizers in soils causes soil degradation to reduce the efficiency of cultivation is repeated. Furthermore, the introduction of phosphorus and nitrogen, which are contained in sewage or wastewater, into the coastal water causes the red tide phenomenon and decomposes in deep water to causes odor in serious case, thereby accelerating water pollution. Therefore, to prevent the water pollution, the phosphorus and nitrogen contained in the sewage or wastewater must be removed before being introduced into rivers or lakes.
Accordingly, the concentration of phosphorus and nitrogen contained in the sewage or wastewater is controlled below a predetermined level and is expected to be more stringently controlled in the future. According to the water quality standards for effluent of sewage treatment plant, which have been published on Apr. 2, 2010 by the Ministry of Environment of Republic of Korea, it has been reported that the regulation of the concentration of total phosphorus would be strengthened from 4 ppm to 0.2 ppm from 2013. Accordingly, it is inevitably necessary to employ a chemical method for additionally removing phosphorus from sewage or wastewater.
The chemical method for removing the nitrogen and phosphorus contained in the sewage or wastewater includes a method of precipitating phosphate ions (PO43−) by adding MgCl2, Al2(SO4)3, FeCl3, etc. to the sewage or wastewater. However, this method requires high cost. Therefore, in order to reduce the processing costs of several hundreds of billions to several trillion Korean Wons, extensive research has been devoted to the crystallization of phosphorus and nitrogen in the form of struvite. As an example thereof, Korean Patent Publication No. 10-1997-20973 and Korean Patent No. 10-572516 disclose a wastewater treatment method for removing phosphorus and nitrogen contained in sewage or wastewater by crystallizing the phosphorus and nitrogen in the form of struvite (i.e., ammonium magnesium phosphate hydrate, NH4MgPO4.6H2O) using seawater or bittern which contains a large amount of Mg2+. Moreover, Korean Patent No. 10-1019200 discloses a method for forming a great amount of struvite by mixing a solution containing phosphate ions (PO43−) with wastewater containing excess nitrogen to be subjected to anaerobic treatment, and supplying a solution containing Mg/Ca ions to the resulting mixture to form struvite. According to this method, the amount of alkaline chemicals used can be minimized, and the nitrogen can be removed from the wastewater containing excess nitrogen.
However, during the removal of phosphorus and nitrogen by struvite crystallization using seawater or bittern in the above manner, materials that inhibit struvite crystallization and excess sodium ions (Na+), which may pollute rivers when the sewage is discharged, as well as Mg2+ are present. Therefore, for the purpose of improving the efficiency of struvite crystallization, various methods for pretreating the materials that inhibit struvite crystallization have been studied. As an example thereof, Korean Patent Publication No. 10-1999-11049 discloses a method for removing phosphoric acid wastewater containing ammoniacal nitrogen in such a manner that slaked lime or quick lime slurry is added to phosphoric acid wastewater containing ammoniacal nitrogen to control the pH to 2.0 to 5.5, fluorine in the wastewater is precipitated and removed such that that only the phosphorus and ammoniacal nitrogen are present in a dissolved state in a supernatant, a Mg compound and a pH adjustment chemical are added to the supernatant to control the pH to 6.0 to 10.5, and the ammoniacal nitrogen is precipitated in the form of struvite and removed. Moreover, Korean Patent Publication No. 10-2006-102762 discloses a method for removing nitrogen by struvite crystallization, which removes components that inhibit struvite crystallization using ferrous salt, Alum and other flocculants and then improves the removal efficiency of nitrogen and phosphorus in a struvite crystallization reactor.
However, although the materials and components that inhibit struvite crystallization can be removed using the methods disclosed in the above patent documents, the use of chemicals such as MgCl2, Mg(OH)2 and MgO as a source of Mg2+ increases the costs, and the use of seawater or bittern as a source of Mg2+ requires the transfer of seawater or bittern to a sewage or wastewater treatment plant, which causes additional logistics costs, thereby reducing the economic efficiency. Moreover, the logistics costs incurred in transferring a large volume of seawater can be reduced when the sewage or wastewater treatment plant is located near the sea, which makes it difficult to apply to all the sewage or wastewater treatment plants.
Therefore, a new method is required to inexpensively provide a source of Mg2+ used for the removal of phosphorus and nitrogen by struvite crystallization.
Meanwhile, iron ore produced at the Shinyemi mine in Gangwon-do, South Korea has an average Fe content of 40% and is a low-grade ore containing MgO and SiO2 as main impurities. During a process of upgrading the low-grade iron ore to be used as an industrial raw material, the Fe contained in the iron ore is recovered, and wastewater produced at this time contains a large amount of Mg2+. Therefore, it is considered that when the iron ore wastewater containing a large amount of Mg2+ is used as a source of Mg2+ in the process for removing phosphorus and nitrogen by struvite crystallization, the cost of wastewater treatment can be reduced.
Therefore, the need for research on the process for removing phosphorus and nitrogen contained in sewage or wastewater using iron ore wastewater containing a large amount of Mg2+ is urgently required.